The invention relates to power-driven conveyors and, more particularly, to sprocket drive systems for modular hinged conveyor belts.
Conventional modular conveyor belts and chains are constructed of modular links, or belt modules, arranged in rows. Spaced-apart hinge eyes extending from each end of the modules include aligned openings. The hinge eyes along one end of a row of modules are interleaved with those of an adjacent row. A pivot rod, or hinge pin, journaled in the aligned openings of end-to-end-connected rows, connects adjacent rows together at hinge joints to form an endless conveyor belt capable of articulating about drive sprockets.
In a typical modular belt conveyor, sprockets are mounted at spaced-apart positions on a drive shaft, which is coupled to a drive motor at one end or both ends of the shaft. Drive shafts are commonly square in cross section or circular with a raised key. Usually all the sprockets are identical with central bores shaped to receive the shaft. Sprocket drive surfaces, such as the leading or trailing edges of sprocket teeth, equally spaced circumferentially around the peripheries of the sprockets, define rows of sprocket drive surfaces aligned parallel to the axis of the drive shaft. The sprocket drive surfaces engage corresponding belt drive surfaces in the underside of the conveyor belt, which, in transition from carryway to returnway, wraps around the drive sprockets. Belt structure in the vicinity of the drive surfaces serves as tracking structure to maintain the belt drive surfaces and the sprocket drive surfaces in correspondence. To keep the belt aligned on the drive shaft and to accommodate variations in belt width with temperature and age, one of the sprockets is typically fixed in position on the shaft and the others are allowed to slide along the shaft. Because the sprockets are identical and the belt drive surfaces are arranged in line across each belt row, the sprocket drive surfaces and the belt drive surfaces are said to be identically timed, or in phase. This means that the drive surfaces on each row (belt or sprocket) define imaginary lines parallel to the axis of the shaft and the width of the conveyor belt. This conventional in-phase sprocket drive system works well in most applications.
In other applications, however, these sprocket drive systems encounter problems. For instance, if the polar moment of inertia of a drive shaft is small, the shaft will twist under a heavy load. One way to increase the polar moment of inertia and to decrease the twist is by using a larger-diameter or otherwise larger cross-sectional drive shaft. But, in many applications, cost, space, or equipment constraints may not allow bigger drive shafts. Twisting of the drive shaft causes the sprocket drive surfaces to get out of phase with each other. Flexible belts are often able to adjust somewhat to out-of-phase sprockets, but stiff belts often cannot. The result is poor or no engagement of sprocket drive surfaces with belt drive surfaces and possible loss of tracking. Continuous poor engagement of drive sprocket with belt accelerates belt failure. This problem is exacerbated in wide belts with long drive shafts in which the total twist and the associated mistiming between sprocket and belt is greater than in narrower belts with short drive shafts.
Thus, there is a need to maintain uniform sprocket drive surface timing at the points of sprocket-to-belt engagement across the width of an entire belt to eliminate the problems caused by the twisting of the drive shaft in sprocket drive systems for modular conveyor belts and to enable the use of smaller, lighter drive shafts.
This need and others are satisfied by the invention, which provides a drive system for a modular conveyor belt. The drive system includes a plurality of belt drive surfaces spaced apart across the width of and opening onto the underside of each row of a modular conveyor belt. A drive shaft is supported for rotation with its axis extending in the width direction of the belt. Sprockets are mounted at spaced-apart locations on the drive shaft. Sprocket drive surfaces are spaced circumferentially around the peripheries of the sprockets and arranged in rows across the sprockets to engage corresponding belt drive surfaces in the belt rows. Means for varying the timing relationship among the drive surfaces between an unloaded condition and a loaded condition are associated with either the sprocket drive surfaces or the belt drive surfaces, or both. Thus, the static timing relationship among the drive surfaces along a row is changed to correct the dynamic timing relationship in the presence of shaft twist.